Vibration

I now have a vibration in my 92 Nauti, but only from about 1000rpm to 1250-1500rpm. After that it goes away. Doesn't occur in neutral. I am 100% certain I didn't hit anything, and there is no prop damage whatsoever. In fact the boat ran fine for 20 minutes prior to having the problem. The shaft does not seem loose, though it does move slightly when I try to force it to move. Strut seems okay too. Tranny fluid is in good shape. Any thoughts?

Thanks
Eric